There are numerous situations in which a small signal needs to be measured and amplified. Sensors for monitoring biosignals (potentials or currents) are one example. Voltage sensing apparatuses for bio-sensing purposes have an important role in the areas of diagnostic analysis of tissue and neuroscience experimentation. Known biosensors comprising small signal voltage sensing apparatus include, for example, neural probes, alternatively named deep-brain implants, or flexible electrode arrays.
Embodiments disclosed herein may be described in connection with one possible use for biopotential measurement. Such biosensors may be used, for example, to measure neural activity at one or more areas of a brain or cardiac activity at one or more areas of a heart. Typically, biopotential sensing apparatuses comprise at least one electrode adapted to detect a voltage present in biological tissue proximate to the electrode.
Measuring circuitry suitable for recording biopotentials is often connected to at least one electrode by a respective connecting wire. Such measuring circuitry may further comprise processing circuitry, for example, a filter or digitization circuitry.
Amplifier circuits are typically implemented as transistor circuits, using one particular transistor technology type. Such technology types typically include standard transistors for operation at core voltages such as 1.2V and high voltage transistors which have thicker gate oxides, for operation at higher voltages such as 3.3V. The standard transistors in scaled technologies have large gate leakage currents that cause high shot noise. This noise can be prohibitive in low frequency analog applications that require very low noise performance. Gate leakage can also cause other issues in an amplifier input stage such as reduced input impedance since excessive current will flow to the input nodes.
US 2006/0290426 discloses a circuit with an input stage implemented using thick gate oxide transistors and a second stage using thin gate oxide transistors. The problem with such a circuit is that the input voltage range which can be processed by the circuit is reduced. In particular, there is a mismatch between the characteristics of the thin and thick gate oxide transistors.
There is therefore a need for an amplifier circuit which can address the issues of gate leakage so that improved signal to noise performance can be achieved, while enabling processing of an input signal over a wide voltage range.